U.S, patent application Ser. No. 878,835, filed Apr. 10, 1986, the subject matter of which is incorporated herein by reference relates inter alia, to a process for coating an electrically conductive substrate, the said substrate being immersed in an aqueous bath based on a cationic synthetic resin binder which is at least in part neutralized with acids and contains amino and/or hydroxyl groups. The said bath contains a mixture of crosslinking agents as well as pigments, fillers, corrosion inhibitors, paint auxiliary substances and, if appropriate, catalysts, as well as organic solvents in an amount of up to 15% by weight of the total weight of the coating, the said substrate being cathodically connected. By the action of direct current, a film is deposited on the substrate, after which the said substrate is removed from the bath and the film is cured by stoving, the mixture of crosslinking agents comprising at least two crosslinking agents of different reactivities based on activated esters and/or polyisocyanates blocked by different blocking agents. The expression "different reactivities" means that the crosslinking reaction commences by heating to temperatures which are separated by at least 10.degree. C. and that crosslinking agent which is first to react is present in an amount of 15 to 60 mol % based on the mixture of the crosslinking agents.
From the start of the electrocoating phase attempts have been made to automate the total painting process. As early as in the '60s, British Pat. No. 1,081,767 described the electrophoretic wet-on-wet painting process. The resins used, for example styrene-butadiene copolymers, failed, however, to produce a paint system with acceptable properties. The voltages reached were of a maximum of 100 V, and consequently insufficient throwing power was possible for the coating of cavities. With the cathodic paint systems available nowadays the outlook is far better. These systems also offer in general the possibility of cathodic multicoat wet-on-wet painting. In a two-coat painting system, consisting of a cathodically applied primer and a cathodically applied top coat, the surface quality which can be attained is, however, highly inadequate.
Consequently there is a need for a process for multicoat cathodic wet-on-wet painting of metallic substrates which would lead to coatings of good surface quality.
In a further embodiment of the main patent it has now been found that a coating system with a good surface quality can be obtained by the process described at the outset, which is characterized in that the first coat is rinsed, one or more further cathodic coats are deposited before stoving and all the coats are cured together after each coat had been rinsed.
The main patent describes in detail methods for measuring the start of the crosslinking reactions. As is also stated there, the increase is viscosity can be defined as the start of the crosslinking reaction.